RFR: 8263477: serviceability/sa/ClhsdbDumpheap.java timed out

[Chris Plummer]

[JDK-8257234](https://bugs.openjdk.java.net/browse/JDK-8257234) added a lot more subtests to this test, so now it sometimes times out. I've double the amount of time given to 240. I'd like to push this as a trivial change.

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Commit messages:
 - Give ClhsdbDumpheap.java a longer timeout since it run a lot more subtests now and sometimes times out.

Changes: https://git.openjdk.java.net/jdk/pull/2981/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2981&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263477
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2981.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2981/head:pull/2981

PR: https://git.openjdk.java.net/jdk/pull/2981